Articles which have organic photochromic material(s) applied to or incorporated therein are characterized in that upon exposure to electromagnetic radiation or to solar light they exhibit a reversible change in color and in light transmission. Once the exposure to the original radiation has been discontinued, the composition returns to its original color, or colorless state. Recently, photochromic plastic materials, most notably, such compositions which may be suitable for the preparation of ophthalmic lenses, films and automotive head lamp lenses have been the focus of attention in the relevant arts. Plastic materials as the medium for the preparation of such lenses allow for the preparation of lighter and thinner lenses than do the traditionally used glass. Also of interest are the applications of photochromic technology to automotive, aircraft transparencies and greenhouse architecture and other glazing applications. It is known that photochromic behavior may be imparted to glass and to certain plastic materials by using inorganic and organic dyes respectively. Photochromic articles prepared from synthetic organic resins such as homopolymers of a poly(allyl carbonate) monomer are known (U.S. Pat. Nos. 4,994,208, 5,246,630, 5,221,721 and 5,200,483.)
U.S. Pat. No. 5,244,602 describes a naphthopyran useful for photochromic polymers and also organic hosts such as polyurethanes for such materials. However, a detailed description of the polyurethane is not given.
In McBain, et al. (U.S. Pat. No. 4,994,208), it was demonstrated that photochromic performance of matrices prepared by the free radical polymerization of polyol (allyl carbonates), e.g., diethylene glycol bis(allyl carbonate), could be improved by the incorporation of 10 to 40 weight percent of an aliphatic polyurethane having terminal ethylenic unsaturation. The polyurethanes described in this reference can be represented by the following expression: EQU D-R-B-A-B-R-D
Here, D represents the terminal functional group containing ethylenic unsaturation, R represents a bivalent alkylene group containing from 1 to 10 carbon atoms, B represents an aliphatic bis carbamate moiety originating from the corresponding aliphatic diisocyanate, and A represents the residue of a saturated aliphatic polyol (e.g., a C.sub.2 -C.sub.6 alkane diol, a polyether, polyester, or polycarbonate diol). In these systems, the polyurethane having terminal ethylenic unsaturation undergoes free radical polymerization with the polyol (allyl carbonate) to form a urethane containing copolymer.
In Selvig (U.S. Pat. No. 5,200,483), it was shown that the compositions described in McBain, et al. could be improved upon (in terms of casting pot life and yellowness) by using appropriate mixtures of allyl and acrylyl groups for the terminal unsaturation in the polyurethanes.
The technology described by Selvig and McBain, et al. requires either a photochromic dye which is resistant to the effects of the peroxy type initiator (see Selvig, Col. 11, lines 53-56) or a secondary processing step, in which the dye is "imbibed" into the finished polymer matrix. (see Selvig, Examples 7 and 10).
Surprisingly, we have now found that polyurethane matrices of the inventive composition are optically clear and have excellent photochromic performance. Since the systems described in our invention do not cure by a free radical mechanism, commercially available photochromic dyes can be added during the casting stage, without interfering in the cure of the polymer or without detriment to the dye's ultimate performance.